This project will test the unineme hypothesis of chromosome structure in mammalian cells by a direct measurement of the molecular weight (Mr) of chromosomal DNA molecules in several mammalian species. Preliminary data, obtained by viscoelastometry (VE), for several species of rodent cells (V79 Chinese hamster lung, L5178Y mouse lymphoblastic leukemia and 9L rat brain tumor) show that mammalian (or at least rodent) DNA molecules have an Mr equal to only 1/4 to 1/8 the average chromatid DNA content. This implies a multimolecular DNA chromosomal architecture. A second preliminary observation in disagreement with the unineme hypothesis is that rodent DNA molecules increase their hydrodynamic (excluded) volume when irradiated with ionizing radiation doses which would be expected to create an average of one double-strand break (DSB) per molecule. Such behavior is expected for circular molecules. No data exist for human cells to determine if they are the sam or different in this regard. However, the different dose dependence for the creation of prematurely condensed chromosome (PCC) fragments suggests that human cells may have linear DNAs. Therefore, data will be obtained to determine if rodent and human chromosomal DNA molecules are linear or circular and their size(s). Two approaches will be used. First, DNA size and shape will be determined on cell lysates; this will yield the size and shape of the largest DNA molecules. Studies will be performed to determine the sensitivity of VE to detect size heterogeneity so that we can determine if all DNA molecules in a nucleus are the same size or different. The second approach will be to sort specific large and small chromosomes by flow cytometry and measure the size (and shape) of their DNAs by VE. Subsidiary studies will be performed to increase the accuracy of DNA size determination in the high ionic strength solutions used to deproteinize the DNA, and to control for any effects of the dyes used for flow sorting of the chromosomes. Thus, these studies will: 1) determine the validity of the unineme model for rodent and human cells, 2) determine the size and shape of the DNA molecules of rodent and human chromosomes, and 3) determine the size of the DNA molecules of specific human and rodent chromosomes.